If you have ideas, you could try making a rough sketch. Either on a free base, or just an attempt at a drawing. It doesn’t have to be good at all, the goal is just to get your ideas on paper. A written description can also work, but I would recommend a drawing. Both could also work. And if you struggle with choosing between some options for details, my recommendation is to just pick one at random. You will get used to it once the design is done.

Then, you can use your rough sketches and such to commission an artist to make you a proper reference sheet.

This is how I did it back when I designed my sona. But others might have other suggestions